Rotation Of CH Research Articles

Molecular structure of nickel(II) and copper(II) N,N′-ethylene-bis(acetylacetoneiminates) MO2N2C12H18 according to gas-phase electron diffraction data and quantum-chemical calculations

The molecular structure of nickel(II) and copper(II) N,N′-ethylene-bis(acetylacetoneiminates), NiO2N2C12H18 and CuO2N2C12H18, at 442(5) K and 425(5)K, respectively. Both molecules have C2 symmetry with a nearly planar MN2O2 coordination site and internuclear distances rh1(M-O) = 1.862(10)/1.923(17) A and rh1(M-N) = 1.879(10)/1.947(18) A for Ni(acacen) and Cu(acacen), respectively. The structure of free molecules is close to the structure of molecules in crystal. The DFT/3LYP quantum-chemical calculations (CEP-31G and 6-31G* basis sets) gave a molecular structure that agreed satisfactorily with the one found in experiment. The low-spin 1A and high-spin 3A states of the Ni(acacen) molecule were considered. It was found that a change in multiplicity caused significant changes in the geometrical and electronic structure of the MN2O2 coordination site. As shown by experiment and calculations for the NiO2N2C12H18 molecule, the low-spin 1A state is the ground state. The internal rotation of CH3(CN) and CH3(CO) methyl groups was studied by the 3LYP/CEP-31G method. It was shown that steric hindrances led to a high rotation barrier of the CH3(CN) group.

Dissociation of the NO–CH 4 van der Waals complex: Binding energy and correlated motion of the molecular fragments

The binding energy of the NO–CH 4 van der Waals complex has been determined by velocity map imaging to be 198 ± 2 cm −1 in the A ∼ state, with a corresponding X ∼ state value of 103 ± 2 cm −1. Individual rotational states of the NO A products have been probed, providing insight into the correlated motion of the NO and CH 4 products. Low rotation of NO correlates with low rotation of CH 4. With increasing rotational excitation in NO there is a corresponding increase in CH 4 rotational excitation. At the highest NO rotation, the CH 4 is forced through conservation of energy to be in low rotational states.

Raman spectrum, conformational stability, barriers to internal rotations and DFT calculations of 1,1,1-trifluoro-propane-2-thione with double-internal-symmetric rotor

The Raman spectrum (3500–100 cm −1) of 1,1,1-trifluoro-propane-2-thione (TFPT), CF 3C(S)CH 3 of the solid phase has been recorded. The internal rotation of CH 3 and/or CF 3 moieties around C C bonds in TFPT allow five hypothetical conformers (C s and C 1 point groups). Aided by quantum chemical (QC) calculations, the Cc conformer is the only stable form (CF 3 and CH 3 groups are eclipsing the C S bond) which contains a planar FCC(S)CH backbone and possess intramolecular hydrogen sulfur interactions,. However, other conformations (with the orientation of sulfur atom being trans to either hydrogen or fluorine atom) are either transition states or not fully converged geometry with “gauche” orientation. Moreover, the calculation were carried out at the level of Becke three Lee-Yan-Parr (B3LYP) parameters up to 6-311++G(d,p) basis sets. The torsional barriers are adequately described by a three-fold potential, V 3 which have been determined utilizing the optimized structural parameters from the B3LYP/6-31G(d) basis set along with potential surface scan. Barriers of 1.28 kcal/mol (448 cm −1) and 1.94 kcal/mol (678 cm −1) were calculated for CH 3 and CF 3 symmetric rotors, respectively. Complete vibrational assignments have been reported for the stable Cc isomer which is supported by normal coordinate analysis and potential energy distributions (PEDs) for all fundamentals. Moreover, equilibrium geometries, vibrational frequencies are compared to the corresponding experimental values of acetone, 1,1,1-trifluoroacetone (TFA), hexafluoroacetone (HFA) and other molecules having the CF 3 moiety whenever appropriate.

Velocity-map imaging study of the photodissociation of acetaldehyde

Velocity-map imaging studies are reported for the photodissociation of acetaldehyde over a range of photolysis wavelengths (317.5-282.5 nm). Images are obtained for both the HCO and CH3 fragments. The mean rotational energy of both fragments increases with photodissociation energy, with a lesser degree of excitation in the CH3 fragment. The CH3 images demonstrate that the CH3 fragments are rotationally aligned with respect to the recoil direction and this is interpreted, and well modeled, on the basis of a propensity for forming CH3 fragments with M approximately K, where M is the projection of the rotational angular momentum along the recoil direction. The origin of the CH3 rotation is conserved motion from the torsional and methyl-rocking modes of the parent molecule. Nonstatistical vibrational distributions for the CH3 fragment are obtained at higher energies.

EPR spectra and rotation of CH3, CH2D, CHD2, and CD3 radicals in solid H2

The EPR spectra of CH3, CH2D, CHD2, and CD3 radicals in a H2 matrix have been observed in the temperature range 1.6–4.2 K. The radicals were obtained by condensation of two gas flows on a cold substrate: deuterium mixed with 2 mol % methane passed through a discharge and pure hydrogen bypassing the discharge. The CD3 and CHD2 spectra were found to be a superposition of two spectra: high- and low-temperature. A transformation of the shape of the CD3 and CHD2 spectra with decreasing sample temperature was observed. This was attributed to a change in the populations of the lowest rotational states of the radicals. Compared to the known results for deuterated methyl radicals in Ar, the present observations suggest the existence of a barrier hindering the rotation of radicals in solid H2.

Vibrational structures of predissociating methylamines (CH3NH2 and CH3ND2) in à states: Free internal rotation of CH3 with respect to NH2

Resonantly-enhanced one-color two-photon (1+1) ionization spectra of jet-cooled methylamines (CH3NH2 and CH3ND2) reveal the vibrational structures of these molecules in predissociative à states. Rotational fine structure is clearly resolved for CH3ND2 at the origin and first wagging vibrational level in the excited state. The spectral linewidth becomes homogeneously broadened to give only vibrationally resolved spectral features for the higher vibrational energy levels of CH3ND2 (Ã). From the spectral analysis of the ÖX transition of CH3ND2, it is found that the methyl moiety rotates nearly freely about the C–N axis with respect to the amino group in the à state, indicating that the removal of an electron from the nonbonding orbital of N is responsible for the free internal rotation. Vibrational levels are only barely resolved in the ÖX excitation spectrum of CH3NH2 due to severe homogeneous line-broadening, indicating ultrashort lifetimes of ∼0.4 ps for predissociating CH3NH2 molecules in the à state. Spectral interpretation of the ÖX excitation spectrum of CH3NH2 is carried out by the comparison with that of CH3ND2, giving the confirmative vibrational assignment of methylamines in à states for the first time. The dramatic difference of CH3NH2 and CH3ND2 in their lifetimes in à states suggests that the major dissociation channel of the excited methylamine may be the N–H (or D) bond dissociation.

Thermal excitation of rotation of the methyl group in chemisorbed acetate on Cu(110)

The adsorption of acetic acid on the Cu(110) surface has been investigated by the time-of-flight electron stimulated desorption ion angular distribution technique by observing H+(D+) ions from the methyl groups. The activated conversion of acetic acid to acetate has been witnessed by observing the formation of oriented acetate species. It has been found that the activation energy for acetate formation varies from 0.39±0.06 eV at low coverage to 1.1±0.1 eV at high coverage, possibly because of acetic acid stabilization at high coverage by hydrogen bonding. Methyl groups are oriented with one C–H bond azimuthally directed in the 〈11̄0〉 direction. At high coverage, the methyl groups are tilted slightly in the 〈11̄0〉 direction due to repulsive acetate–acetate interactions. The activated rotation of the methyl groups on adsorbed acetate is clearly observed by the electron stimulated desorption ion angular distribution method and the barrier height for CH3 rotation is estimated to be 12±6 meV, and for CD3 groups to be 7±4 meV. The small inverse D isotope effect is postulated to be caused by higher amplitude CH3 wagging motions compared to CD3, which permit slightly higher hyperconjugation for CH3 as this group interacts with the anchoring carboxyl group.

Rotational Tunnelling in Methyl Metal Compounds X2(CH3)4 with X = As, Sb, Bi

Abstract Rotational excitations of methyl groups attached to metal atoms of the fifth main group have been investigated in X2(CH3)4 crystals with X = As, Sb, Bi by 1H nuclear magnetic resonance (NMR) relaxation measurements and inelastic neutron scattering (INS). CH3 rotation has been found to be rather weakly hindered (activation energies between 1.7 kJ/mol and 5.8 kJ/mol) so that quantum effects are important to describe the results. Tunnel splittings between 0.1 μeV and 23 μeV have been observed. Non-equivalent methyl groups in tetramethyldistibane and tetramethyldibismuthane have been identified by both INS and NMR. A consistent description of the T1 data is possible by Haupt's equation. The rotational potentials could be derived and relations to the crystal structure have been discussed.

Rovibrational distributions of CH(A 2Δ,B 2Σ−) produced in energy-transfer reactions from Ar(3P2), Kr(3P2), and Xe(3P2) atoms to CH3 radical

Energy-transfer reactions from Ar(3P2), Kr(3P2), and Xe(3P2) to CH3 radical have been studied by observing emission spectra from excited fragments in the flowing afterglow. CH3 radicals were generated by the F+CH4 reaction. The CH(A 2Δ–X 2Πr:v′=0−2) and CH(B 2Σ−–X 2Πr:v′=0) emission systems were observed in the Ar(3P2) reaction, while only CH(A–X:v′=0,1) emission system was found in the Kr(3P2) and Xe(3P2) reactions. The nascent rovibrational distributions of CH(A:v′=0–2) were N0:N1:N2 =100(T0 =3400±400 K):28±5(T1 =1700±400 K):4±1(T2 =700±300 K) in the Ar(3P2) reaction and 100(T0 =1000±250 K):<5(T1 <800 K):0 in the Kr(3P2) and Xe(3P2) reactions. The rotational distribution of CH(B:v′=0) in the Ar(3P2) reaction was reproduced by a single Boltzmann temperature of 2800±300 K. The average fractions of total available energies channeled into vibration and rotation of CH(A,B) were less than 15% for all cases, suggesting that most of the available energies was deposited as relative translational energy of products and/or rovibrational energy of H2. The observed rovibrational distributions of CH(A) were colder than those predicted from statistical theories including and excluding the conservation of total angular momentum. The best agreement between the observed and statistical distributions was obtained for the mechanism giving CH(A,B) in two-body dissociation steps by assuming that 78–92% of the total available energy is released as kinetic energy in the first step, Rg(3P2)+CH3→CH*3+Rg, then the rest remains in the precursor CH*3 state as an internal energy.

A b i n i t i o potential energy surfaces and trajectory studies of A-band photodissociation dynamics: CH3I*→CH3+I and CH3+I*

A b initio contracted spin–orbit configuration interaction (SOCI) calculations have been carried out to obtain potential energy surfaces of 3Q0 and 1Q1 excited states of methyl iodide as functions of all the geometrical parameters except for the three C–H stretches. The results are fitted to six-dimensional diabatic potential functions and their couplings. Classical trajectory calculations have been performed using these potential functions. The rotation of the CH3 product in the I channel has been calculated to be perpendicular to the top axis and to have a peak at N=5 and extend up to N=8, whereas it is cold in the I* channel, in good agreement with recent experiments. The CH3 rotation is excited by the time trajectories arrive at the conical intersection region; this excitation is retained in the I-channel product because the 1Q1 surface has a small bending force constant outside the conical intersection, whereas it is damped in the I* channel because 3Q0 still has a large bending force constant. The calculated distribution in the ν2 umbrella vibrational mode of the CH3 product is hot and has a peak at v=2 for the I channel, and is cool for the I* channel, in good agreement with recent experiments. This channel selectivity is due to the difference in the preferred structure of the CH3 group outside the conical intersection region; while the 3Q0 surface prefers a bent CH3 until the CH3–I distance becomes very large, 1Q1 wants a planar CH3. The location of conical intersection and the ground-excited energy difference there are in good agreement with those deduced from experiment if a dynamical effect is taken into account.

Rotational isomerism, molecular motion and hydrogen bonding as studies by supersonic jet spectroscopy

The application of supersonic jet spectroscopy to rotational isomerism and related problems is described. It is demonstrated by taking o-substituted phenols and substituted benzaldehydes as examples that electronic spectra of jet-cooled molecules are very powerful for the study of the rotational isomerism of large polyatomic molecules. The identification and characterization of rotational isomers can be made by the effects of inter- and intramolecular hydrogen bondings on electronic spectra and relaxation process and by the potential of the internal rotation of CH 3 group.

Methyl rotation, vibration, and alignment from a multiphoton ionization study of the 266 nm photodissociation of methyl iodide

The photodissociation dynamics of CH3I and CD3I have been examined by using multiphoton ionization to probe the CH3, CD3, I(≡5 2P3/2) and I*(≡I 5 2P1/2) photoproducts. The parent compounds were cooled in a supersonic expansion, collimated into a molecular beam, and dissociated at 266 nm. For the CD3I dissociation, the ratio of CD3(v=0)/(v=2) was estimated to be about 1.1, with multiple determinations ranging from 0.47–2.1. The quantum number v here denotes the nascent excitation of the ν2 ‘‘umbrella’’ mode. Measurements of the CD3(v=1) and (v=3) vibronic bands indicated that the (v=1)/(v=3) ratio is greater than unity, with some measurements suggesting values as large as 10. A value for the CH3(v=0)/(v=2) ratio from dissociation of CH3I could not be estimated, although it was clearly larger than that for CD3. The CH3(v=0) and CD3(v=0) products from this dissociation are fit by 120±30 K and 105±30 K rotational distributions, respectively. The dissociation mechanism produces alignment in the molecular frame such that there is a strong preference for K=0 (rotation perpendicular to the top axis). Assuming that the relative velocity vector lies along the CH3 C3 axis, the velocity and rotation vectors tend to be perpendicular. It is likely that K=0 molecular frame alignment is produced in photodissociation through both the I and I* channels.

ChemInform Abstract: Cycloalkylmethyl Radicals. Part 6. The Unexpectedly High Barrier to the Rotation of Axial CH· 2 Groups in Cyclohexylmethyl Radicals

ChemInformVolume 20, Issue 7 Preparative Organic Chemistry ChemInform Abstract: Cycloalkylmethyl Radicals. Part 6. The Unexpectedly High Barrier to the Rotation of Axial CH· 2 Groups in Cyclohexylmethyl Radicals L. HUGHES, L. HUGHES Dep. Chem., Univ. St. Andrews, St. Andrews, Fife, KY16 9ST, UKSearch for more papers by this authorK. U. INGOLD, K. U. INGOLD Dep. Chem., Univ. St. Andrews, St. Andrews, Fife, KY16 9ST, UKSearch for more papers by this authorJ. C. WALTON, J. C. WALTON Dep. Chem., Univ. St. Andrews, St. Andrews, Fife, KY16 9ST, UKSearch for more papers by this author L. HUGHES, L. HUGHES Dep. Chem., Univ. St. Andrews, St. Andrews, Fife, KY16 9ST, UKSearch for more papers by this authorK. U. INGOLD, K. U. INGOLD Dep. Chem., Univ. St. Andrews, St. Andrews, Fife, KY16 9ST, UKSearch for more papers by this authorJ. C. WALTON, J. C. WALTON Dep. Chem., Univ. St. Andrews, St. Andrews, Fife, KY16 9ST, UKSearch for more papers by this author First published: February 14, 1989 https://doi.org/10.1002/chin.198907120Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume20, Issue7February 14, 1989 RelatedInformation

Studies of magnetic resonance phenomena in polymers. II. Molecular motions in poly(methyl methacrylate) as studied by spin‐probe and spin‐label methods

AbstractThin films of spin‐probed and spin‐labeled poly(methyl methacrylate) (PMMA) have been examined by electron spin resonance (ESR) in the temperature range of 77–520°K. The rotational correlation times of nitroxides used as spin probes and labels have been determined as a function of temperature from which activation energies are also determined. The nitroxide rotational times are found to strongly correlate with local segmental and side‐chain motions of the host PMMA matrix. Five discrete molecular motions are detected in PMMA along with their activation energies as measured: side‐chain CH3 rotation (1 Kcal/mol), main‐chain CH3 rotation (2 Kcal/mol), ester side‐chain COOCH3 rotation around the CC bond (4 Kcal/mol), main‐chain CC bond rotation (6 Kcal/mol), and side‐chain OCH3 rotation around the CO bond (18 Kcal/mol). The activation energies determined by ESR are consistent with the potential‐energy barriers calculated theoretically for various rotations in PMMA. It is concluded that the probe and label rotational motions do respond to localized, small‐scale segmental and side‐chain motions of host polymers but are relatively ineffective in response to the large‐scale segmental motion with an activation energy larger than 20 Kcal/mol in the case of PMMA.

Molecular motions in filled polydimethylsiloxanes

Molecular motions in filled polydimethylsiloxanes (PDMS) have been investigated. Frequencies of the most low temperature motions (rotation of CH 3 groups about the SiC bond) are scarcely dependent on the filler content. Motions of chain units are severely inhibited in an adsorption layer of thickness ∼ 8 A ̊ . Motions of chain fragments adjoining the adsorption layer are also inhibited to some extent. In PDMS with a high loading of filler a fraction of the nonadsorbed units are highly mobile below T g, apparently on account of defects in the packing of individual chain fragments. Frequencies of rapid reoriented motions of nonadsorbed units some distance away from filler particles approximate to those of motions in the unfilled PDMS. At the same time slow translational motions of the units are inhibited.

Laser induced fluorescence spectroscopy of van der Waals complexes of aniline with N 2, H 2, and CH 4 formed in a supersonic free jet

The laser induced fluorescence spectra for the Ã( 1 B 2) X̃( 1 A 1) transition of van der Waals (vdW) complexes of aniline with N 2, H 2, and CH 4 have been observed. Based on the analysis of the rotational structure of the spectra, it is suggested that two vdW conformers exist for the N 2aniline complex though only one conformer is identified for the other complexes. In the electronically excited state of the CH 4aniline complex, energy level splittings are observed and attributed to the intramolecular rotation of CH 4.

Interpretation of the vibrational spectra of methylglyoxal and biacetyl in their first singlet excited electronic states

Laser-induced fluorescence spectra for the first allowed electronic transition (22125 cm−1) of methylglyoxal (CH3COCHO) and its perdeutero analog (CD3COCDO) in a supersonic nozzle beam are quantitatively represented assuming that the potential function governing the CH3(CD3) rotation is changed during the transition. In the excited state the potential function is ternary (V1 = 95 (1 + cos 3θ)cm−1) as in the fundamental state (V0 = 134.5 (1 - cos 3θ)cm−1), but the minima are shifted by an angle of π/3. The spectrum of biacetyl (CH3COCH3CO) can be reproduced assuming two uncoupled methyl groups undergoing similar conformational changes during the electronic transition (the estimated potential function is V1 = 117.5 (1 + cos 3θ) cm−1 for each methyl group), in perfect agreement with the most recent assignment of the 0-0 transition. These results are consistent with ab initio calculations for the fundamental and first excited singlet states.

NMR Studies on Intercalation Compounds of Layered Chalcogenides with Methylamines and Ammonia

Abstract NMR wideline and pulse techniques have been used to study orientation and molecular motions of the intercalation compounds of niobium disulfide with monolayers of mono-, di-, and trimethylamine, tetramethylammonium and ammonia. Specific information has been obtained from the 1H and 2D spectra and from spin-lattice relaxation measurements of the selectively deuterated monomethylamines. Models for the arrangement of the various molecules within the layers and for the motions have been derived. One-dimensional rotation of CH3 and NH3 groups, rapid intermolecular exchange of NH, NH2 and NH3 protons, as well as translational and reorientational motions of whole molecules are of importance. Interpretation of spin-lattice relaxation rates at various temperatures and frequencies yields details of some of these motion and numerical values for the activation energies.

ChemInform Abstract: THEORETICAL STUDY OF THE BARRIERS TO INTERNAL ROTATION IN DIMETHYL ETHER, METHOXYSILANE AND DISILOXANE

Abstract The barriers to internal rotation have been computed by ab initio methods for CH3OCH3, CH3OSiH3 and SiH3OSiH3. It is shown that minimal basis set results agree fairly well with experimental results for CH3OCH3 and to a lesser extent for SiH3OSiH3. To predict a correct ordering of the CH3 and SiH3 rotation barriers in CH3OSiH3, a split valence basis set has to be used. Attention is also paid to the influence of geometry optimization on the barriers to internal rotation.

Structural relaxation upon internal rotation in cis-methyl nitrite, CH 3 ONO

The ab initio SCF gradient method has been used to obtain changes of bond lengths and valence angles upon internal rotation of CH 3 and NO groups in cis-methyl nitrite. The data for methyl torsion are confirmed by comparison of calculated and observed shifts of rotational constants in the first methyl torsional state.